Conventionally, the mainstream of semiconductor elements used in power converters was pressure-contact semiconductors such as GTO thyristors and GCTs. In recent years, however, self-arc-extinguishing elements such as IGBTs have become mainstream and there have been an increasing number of instances of use of a plurality of self-arc-extinguishing elements connected in series. For example, a technique of forming one arm by connecting a plurality of self-arc-extinguishing elements in series is known. The self-arc-extinguishing elements, e.g., IGBTs have a switching speed higher than those of the above-mentioned pressure-contact semiconductors. There is a problem that when the arm is turned off, an overvoltage is applied to a particular one or ones of the semiconductor elements if a voltage is not evenly applied to the self-arc-extinguishing elements constituting the arm. Regarding this, a technique disclosed in Patent Literature 1 shown below (JP 2004-140891 A) may be mentioned, which is a technique to protect a semiconductor element by clamping an overvoltage applied to main electrodes of the semiconductor element.